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Speedy cannabis spit test could spot people driving while high

Jointed-up thinking?

Phanie / Alamy Stock Photo

By Joshua Sokol

An erratic driver is pulled over by the police. The officer smells a hint of marijuana, so dabs a cotton swab in the driver’s mouth to collect some saliva. Just 3 minutes later, still by the side of the road, the result comes back: every millilitre of spit contains 5 nanograms of THC, weed’s active ingredient.

This scenario may soon be commonplace thanks to a new test developed in the lab of Shan Wang at Stanford University in California.

Wang’s technology uses nanoparticles that are shaped to fit like a lock-and-key to either THC or to reagents attached to a surface. With no THC molecules around, they connect to the reagent molecules, creating an electromagnetic distortion a sensor can measure. Add in THC and there are fewer distortions. “The more THC in the saliva, the less signal we detect,” Wang says.

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The sensor connects to a smartphone through Bluetooth, making it easy to use on the go – there’s no need to take samples back to the lab. “I think field testing is really the next step,” Wang says. “We have to make the device more user-friendly to the law enforcement officers.”

Wang’s group is one of many developing breathalysers, sweat measurers and other saliva tests for THC, but a better scientific understanding of the drug is needed to help formulate drug-driving legislation.

Twenty-three US states now allow cannabis use for medical purposes, and four allow purely recreational use. But no one quite knows exactly what to do about driving regulations. Twelve states don’t allow drivers any THC, and five more have set thresholds of 5, 2, 1 or nanograms per millilitre.

Everyone is different

That spread of thresholds reflects the fact that, unlike with the concentration of alcohol in the blood, the concentration of THC in bodily fluids doesn’t seem to be a good indicator of how impaired a person is. The effect of a given amount can vary greatly from person to person. The same is true of alcohol, but it has been much better studied, providing a good threshold to base laws on.

Two developments will help to clarify the situation, says an expert at the US National Highway Traffic Safety Administration, who wants to remain anonymous to avoid the appearance of endorsing a particular test. Accurate testing devices like Wang’s are needed to provide more data, as is more work to find the correlation between measurements and bad driving.

With that in mind, Wang’s idea to go after saliva with nanoparticles has promise. “It’s a convenient bodily fluid to sample,” the expert says. “If it can be analysed quickly and with known accuracy, and if that measurement can be correlated well with actual impairment, then you’ve got something quite useful.”